1. Field of the Invention
This invention relates to a drop-out detection circuit, and more particularly, to a circuit highly suited for detecting drop-out of angle modulated signals.
2. Description of the Related Art
In reproducing from a recording medium such signals as video signals with a reproducing apparatus, some parts of the signals often drop out due to various causes. The apparatus of this kind is, therefore, generally provided with some arrangement to compensate for such signal drop-out.
To have signal drop-out adequately compensated for, it is necessary to accurately detect the drop-out. There have been proposed various drop-out detecting methods. FIG. 1 of the accompanying drawings shows an example of the conventional drop-out detection circuit adapted for detecting the drop-out of frequency modulated signals in particular. Referring to FIG. 1, an input terminal 31 receives, for example, a frequency modulated luminance signal S-31 which is as shown at a part S-31 in FIG. 2. The signal S-31 is amplified by the amplifier 32 and then is changed into a signal of a given level by a limiter 33 as shown at a part S-32, in FIG. 2. The signal S-32 which is produced from the limiter 33, is applied to a fall synchronizing type retriggerable monostable multivibrator (hereinafter referred to as monostable multivibrator) 34. With the time constant T of the monostable multivibrator 34 set at a given value by a resistor 34a and a capacitor 34b, the output level of the monostable multivibrator 34 becomes low in the event of a long drop-out exceeding the time constant T, as shown at a part S-33 in FIG. 2. Then, the output S-33 of the monostable multivibrator 34 is obtained from an output terminal 35 as a drop-out detection signal.
However, in cases where a signal of a broad band is to be processed with the conventional device of the above-stated kind, by arranging to make the device capable of detecting also a very short drop-out such as 1 to 2 .mu.s, for example, the operation of the device becomes extremely unstable. In that instance, the device often fails to perfectly follow the drop-out occurring for such a short period of time. In the case of the device arranged in the manner as mentioned above, the device may be enabled to detect such a short drop-out by arranging the monostable multivibrator 34 to have an extremely small time constant T. However, an unstable operation generally results from making the time constant of a monostable multivibrator extremely small and thus prevents the monostable multivibrator from accurately following a trigger signal.
Further, in the case of such a short period of time, it is also difficult to select and accurately adjust the time constant set by a capacitor and a resistor. Besides, since the device is provided with a capacitor, its operation tends to become unstable due to changes in temperature. It has been another problem that the conventional device does not readily permit use of an IC.